The process of electrodeposition, in various steps of operation, imposes a contradictory behavior on the paint binders being deposited. On the one hand, the binders or the components of the binders should have a low viscosity, with low quantities of organic solvents, in order to permit ease of pigment loading and processing of the electrodeposition paints. On the other hand, a sufficiently high molecular weight and thus a high viscosity is a preliminary condition for the required resistance characteristics of the stoved film. There is a similar contradiction between a desired good solubility and dispersibility in water of the binder as an essential carrier medium on electrodeposition and the formation of a hydrophobic film after protonation on electrodeposition and after crosslinking. Contradictory exigencies also apply to the formation of a high electric film resistance on deposition in order to achieve a high throwing power versus high film thickness which can only be achieved with a low electric film resistance.
The literature discloses numerous attempts to achieve a compromise between optimum solubility and dispersibility of the protonized binder and the deposition characteristics and the properties of the crosslinked paint film including through the use of a variety of modifications of bisphenol A or phenol-novolak-epoxy resin-amine adducts. In U.S. Pat. Nos. 4,104,147 and 4,148,772, for example, the epoxy resin based on bisphenol A and epichlorohydrin, prior to the reaction with an amine, is reacted with a polytetramethylene glycol to prolong the chain and to thus introduce a hydrophilic segment. Also, U.S. Pat. Nos. 3,839,352 and 4,035,275 propose a chain prolongation with polypropylene glycol prior to the reaction with an amine. EP-A2-00 74 634 discloses chain prolongation of low molecular weight bisphenol A epoxy resins with bisphenol A ethylene oxide adducts.
The essential disadvantage of all these prior methods is the reduced controllability of the chain prolonging reactions, because self-condensation of the epoxy resin cannot be prevented. Even if the intermediates have the theoretical epoxy value, the presence of free polyols cannot be excluded, entailing an essential influence on various properties of the binder.
Epoxy-amine adducts based on polyoxyalkylene glycidyl ethers, disclosed as for example in U.S. Pat. No. 4,035,275, show extremely good water-solubility. However, electrodeposition of these binders to give satisfactory films is extremely difficult. Also, the resistance characteristics are very unsatisfactory, as could be expected.
Cationic modified epoxy resins soluble in water at a pH-value of over 7 can be obtained by introducing quarternary ammonium groups, i.e., by reaction of the epoxy groups with tertiary amines in the presence of acids and/or water. Products of this type, as are disclosed for example in U.S. Pat. No. 4,035,275, proved suitable as the sole binders for practical use only in case of reduced requirements and are primarily used as addition binders only.